Add new signals and logic to handle open directories in new windows or tabs

The new signals allow dolphin to open the folder in a new active tab if ctrl+shift+left-click or shift+middle-click is pressed and a new window if shift+left-click is pressed instead.

It is needed by [Dolphin] Enable Ctrl/Shift-Click to open folder in a new tab/window

PS: I didn't know exactly what to do with the @since field in the documentation (is it somehow automated or is it done by a maintainer?), so I just put there the one I think will be the next KIO/Frameworks version.